On the second " day,"
the atmosphere evidently continued to clear, creating a space between the thick clouds above and the ocean below.
On the fourth " day,"
the atmosphere had gradually cleared to such an extent that the sun and the moon were made to appear..
" in the expanse of the heavens."
{ Genesis 1:14-16 }

In other words, from the perspective of a person on earth, the sun and the moon began to be discernible.
Theses events happened gradually..
The Genesis account also relates that as the atmosphere continued to clear, flying creatures--- including
insects and membrane winged creatures--- started to appear on the fifth " day."
However , the Bible indicates that during the sixth " day,"
God was still in the process of " forming from the ground every wild beast of the field and every flying creature of the heavens."
{ Genesis 2:19 }

Clearly, the Bible's language makes room for the possibility of some major events during each " day," or creative period ,
to have occured gradually rather then instantly, perhaps some of them even lasting into the following creative " days."